1. Field of the Invention
The present invention relates generally to a zoom lens and more particularly to a zoom lens for a camera.
2. Discussion of the Background
There has been a demand in recent years for a lens shutter camera with a variable angle of view, and to meet such a demand, bifocal cameras which can switch to a telephoto mode have begun to be in use.
Another demand which has arisen is for a zoom lens with its focal length continuously variable. Lens shutter cameras do not require a long back focus as is the case with the zoom lens of a conventional single-lens reflex camera, but need a lens with a short distance from a first lens surface to an image point. Zoom lenses have also heretofore been used on video cameras or the like. However, zoom lenses of smaller size are desired for such use.
Zoom lenses which have been proposed to meet the above demands are disclosed in Japanese Laid-Open Patent Publication Nos. 57-201213, 60-48009, 60-170816, 60-191216, and 60-191217, for example. These proposed zoom lenses are referred to as two-group zoom lenses.
The two-group zoom lenses includes a first lens group having positive refracting power and a second lens group having negative refracting power. Upon zooming, the distance between the first and second lens groups is varied, and so is the back focus.
The zoom lens of this type fails to achieve good lens performance over an entire zooming range because, upon zooming, the path of a light ray passing through the second lens group is changed greatly resulting in large variations in aberrations of the second lens group such as astigmatism, coma, and chromatic aberration. The distortion of the image tends to be increased while being positive, and is liable to be of the pin-cushion type which the user usually dislikes. The exit pupile is apt to become close to the image plane, so that off-axis light rays fall on the image plane at a large angle. Therefore, if there were irregularities on the film surface, the produced image would be greatly distorted. Since the light is applied to the film surface at the large angle, light reflections from the film surface are increased, causing a reduction in the amount of light at the peripheral edge of the field.
Japanese Laid-Open Patent Publication No. 58-199312 discloses a three-group zoom lens comprising a first lens group having positive refracting power, a second lens group having negative refracting power, and a third lens group having positive refracting power. Upon zooming movement of the lens from a wide-angle setting to a telephoto setting, the distance between the first and second lens groups decreases, whereas the distance between the second and third lens groups increases and so does the back focus.
With the third lens group being movable, the zooming mechanism is complex, and the camera body itself is large in size inasmuch as the zooming mechanism must be disposed outwardly of the third lens group which is of the greatest lens diameter. The refracting power of the second lens group is required to be increased to prevent the distance between the first and second lens groups from being excessively small in the telephoto setting. This however increases the amount of aberration produced by the second lens group, and increased amount of aberration cannot sufficiently be corrected.